Presence of arsenic (As) and chromium metal ions in drinking water has become the issue of global concern. Long-term exposure to even low concentrations of arsenic in the drinking water may cause skin, lung or prostrate cancer and cardiovascular, pulmonary, immunological and neurological disorder [Environment Health Criteria 224, Arsenic and Arsenic Compounds, Second edition, World Health Organization 2001; S. Shevade and R. G. Ford. Use of synthetic zeolites for arsenate removal from pollutant water. Water Res. 38(14-15), 3197 (2004)].
At present there is no effective medicine available, which can treat disease, causes by arsenic and chromium, so use of arsenic and chromium free water can help the affected person to get rid of the symptoms of arsenic and chromium toxicity. Hence, the requirement of arsenic and chromium free water is urgently desired to mitigate arsenic and chromium toxicity and protection of the health of human beings living in the areas affected by arsenic and chromium contamination.
World Health Organization (WHO) & Environment Protection Agency (EPA) has established a level of 10 μg/L arsenic and 50 μg/L of chromium in drinking water from January 2006. But in several countries like Bangladesh, India, arsenic and chromium concentration in the drinking water can be as high as 500 μg/L or more. The reduction of arsenic and chromium from such high concentrations and made it potable as per WHO prescribe limit is a very challenging task.
Arsenic occurs in rocks, soil, water and air in −3, 0, +3 and +5 valence state. It is widely distributed having average concentration of 2 mg/kg. The burning of fossil fuels, refining of petroleum mining, smelting of metals like Zn, Cu, Ni, and Pb, are major anthropogenic sources for arsenic contamination in air, water and soil.
Therefore high toxicity and widespread occurrence created a pressing need for effective monitoring, measurement and remediation of arsenic in soil and groundwater. The effect and degree of toxicity of arsenic depends on its inorganic or organic forms and oxidation state. Inorganic arsenicals are more toxic than organic arsenicals and in inorganic arsenicals trivalent form is more toxic than the pentavalent form.
Reference may be made to Y. Lee, I. H. Um and J. Yoon, Arsenic (III) oxidation by iron(VI) (ferrate) and subsequent removal of arsenic (V) by iron (III) coagulation. Environ. Sci. Technol. 37(24), 5750 (2003); B. Daus. R. Wennrich and H. Weiss, Sorption materials for arsenic removal from water: a comparative study. Water Res. 38(12), 2948 (2004); S. Bang, G. P. Korfiatis and X. Meng, Removal of arsenic from water by zero-valent iron. J. Hazard Mater. 121(1-3), 61 (2005); Y. S. Shen. Study of arsenic removal from drinking water. J. American Water Works Association, 65(8), 543 (1973) and A. Joshi and M. Chaudhary. Removal of arsenic from groundwater by iron-oxide-coated sand. ASCE J. Environ. Engineering. 122(8), 769 (1996), which discloses several methods for the removal of arsenic from contaminated water to the consumable limit.
In U.S. Pat. No. 4,566,975, heavy metals such as arsenic are removed in a two step process which involves an alkaline precipitation carried out at a pH of at least about 8 and using ferrous sulfate as an additive.
In U.S. Pat. No. 4,880,510, the electrolytic cell has been used to remove color impurities such as dyes from wastewater solution. The ferrous iron generated at the anode reacts with hydroxide ion to form an iron complex or compound, which further was found to react with or otherwise remove the color bodies from aqueous media as an insoluble precipitate.
In U.S. Pat. No. 4,490,257, contaminants are removed by electrolysis process. The electrodes are resistant to corrosion.
In U.S. Pat. No. 5,043,080, contaminated groundwater is treated with hydrogen peroxide and transition metal ions at an acid pH in the presence of ultraviolet light. The main object, however, is the removal of organic contaminants rather than heavy metals.
In U.S. Pat. No. 4,163,716, it was recognized that heavy metals and color bodies from dye house affluent could be removed with ferrous ions supplied by iron electrodes with the ferrous ion oxidizing to the ferric state by use of an oxidizing agent such as hydrogen peroxide. At a pH of between 7 and 9, heavy metals and traces of color adhere to the ferric hydroxide floc, which then may be removed. This process also involves a pH adjustment from a reaction pH of below 6.5 to a pH of from 7 to 9 to achieve removal of color particles.
The common technologies used for removal of arsenic are oxidation, co-precipitation, adsorption onto sorptive media, ion exchange resin and membrane techniques etc. Presently, various materials like activated carbon (AC), zirconium coated activated carbon (Zr-AC) [B. Daus, R. Wennrich and H. Weiss, Sorption materials for arsenic removal from water: a comparative study. Water Res. 38(12), 2948 (2004)]; iron hydroxide [W. Wang, D. Bejan and N. J. Bunco, Removal of arsenic from synthetic acid mine drainage by electrochemical pH adjustment and co-precipitation with iron hydroxide. Environ. Sci. Technol. 37(19), 4500 (2003)]; iron (II) and iron (III) oxides [L. C. Roberts, S. J. Hug, T. Ruettimann, M. Billah, A. W. Khan and M. T. Rahman; Arsenic removal with iron (II) and iron (III) in waters with high silicate and phosphate concentrations, Environ. Sci. Technol. 38(1), 307 (2004)], sand and zero-valent iron [O. X. Leupin and S. J. Hug. Oxidation and removal of arsenic(III) from aerated groundwater by filtration through sand and zero-valent iron; Water Res. 39(9), 1729 (2005)], hardened paste of Portland cement [Kundu, S. S. Kavalakatt, A. Pal, S. K. Ghosh, M. Mandal and T. Pal; Removal of arsenic using hardened paste of Portland cement: batch adsorption and column study. Water Res. 38(17), 3780 (2004)]: iron oxide coated polymers [A. Katasoyiannis and A. I. Zouboulis, Removal of arsenic from contaminated water sources by sorption onto iron-oxide-coated polymeric materials, Water Res. 36(20), 5141 (2002)]; biological systems (bacteria) [A. Katasoyiannis and A. I. Zouboulis; [Application of biological processes for the removal of arsenic from groundwater] Water Res. 38(1), 17 (2004)] has been removed arsenic from contaminated water through biological process and could be used for drinking and other household utilities.
The above materials and methods are effective and reduce arsenic concentration in the potable water up to acceptable limits. But these materials and methods have their own advantages and disadvantages like, oxidation process is very simple and low cost but it is very slow and removes only a part of the arsenic, co-precipitation by alum or iron is again simple and low capital arrangement but it produces toxic sludge's and pre-oxidation is required to start the reaction.
The use of iron or iron oxide for removing arsenic is dominative as it is very cheap, highly effective and can purify large volume of water. In this process, arsenite (As3+) species is first oxidized to arsenate (As5+) in the presence of atmospheric oxygen, or Ozone or free chlorine. Reference may be made to G. Hering, P. Y. Chen. J. A. Wilkie, M. Elimelech and S. Liang. Arsenic removal by ferric chloride. J. American Water Works Association; 88(4), 155 (1996). Wherein the arsenate species got adsorbed over the surface of iron oxide during filtration and are removed from the contaminated water. Roberts L. C., et al. have used Fe (II) and Fe (III) to remove arsenic from water with high silicate and phosphate concentrations.
Reference may be made to Daus B. et al. Water Res. 2004 July; 38-(12): 2948-54 that has proved that arsenite and arsenate can be removed from water using Activated carbon (AC), zirconium loaded Activated carbon with other materials successfully.
Reference may be made to Water Res. 2003 May; 37(10): 2478-88, wherein the arsenic was effectively removed by steel manufacturing byproducts like evaporation cooler dust (ECD), oxygen gas sludge (OGS), and basic oxygen furnace slag (BOFS).
Reference may be made to Bang S. et al. J. Hazard Mater. 2005 May 20, 121(1-3): 61-67, who has reported effect of dissolved oxygen and pH on the removal of Arsenic from water and concluded that at pH 6 that arsenate removal (99.8%) was faster than arsenite (82.6%) and more dissolved oxygen and low pH increases the rate of iron corrosion and leads to the formation of iron hydroxide, which ultimately adsorbs arsenic from the solution.
Reference may be made to S. Kundu, S. S. Kavalakatt, A. Pal, S. K. Ghosh, M. Mandal, and T. Pal, Water Res. 2004 October; 38(17): 3780-90 wherein Portland cement (HPPC) paste has been used as adsorbent for the removal of arsenic from water and have shown that 95% arsenate and 88% Arsenite can be removed easily.
Reference may be made to Sarkar, A. et al. Water Res. 2005 May; 39(10): 2196-206 wherein activated alumina has been used as adsorbent for arsenic removal from drinking water.
Reference may be made to Bang S. et al. Chemosphere. (2005) July; 60(3): 389-97 wherein granular titanium dioxide (TiO2) has been used for the removal of arsenic from groundwater. Reference may be made to Oklahoma State University, USA, Advanced ceramic reports; Issue: August 2004, page: 6 wherein Porous Zinc oxide heads has been used to remove arsenic from the contaminated water.
Chromium is a common heavy metal contaminant of water supplies, largely arising from the textile, leather and wood production industries. The metal industry mainly discharged trivalent chromium. Hexavalent chromium in industrial wastewater mainly originates from tanning and painting. Chromium may be applied as a catalyser, in wood impregnation, in audio and video production and in lasers. Chromite is the starting product for inflammable material and chemical production. Levels of chromium in drinking water have been controlled in the past by expensive, often toxic chemical based cleansing procedures.
Trivalent chromium is a dietary requirement for a number of organisms as trivalent chromium is an essential trace element for humans and with insulin it removes glucose from blood and also plays a vital role in fat metabolism.
But hexavalent chromium is very toxic to flora and fauna. The human body contains approximately 0.03 ppm of chromium. Daily intake of chromium depends upon feed and levels, and is usually approximately 15-200 μg, but may be as high as 1 mg. The Placenta is the organ having highest chromium amounts. Chromium deficits may enhance diabetes symptoms. Chromium can also be found in RNA. Chromium deficits are very rare, and chromium feed supplements is not often applied. Chromium (III) toxicity is unlikely, at least when it is taken up through food and drinking water. It may even improve health, and cure neuropathy and encephalopathy. Hexavalent chromium is known for its negative health and environmental impact. It causes allergic and asthmatic reactions and it is 1000 times more toxic than trivalent chromium. Exposure to hexavalent chromium causes diarrhoea, stomach and intestinal bleeding, cramps, paralysis and liver and kidney damage. The hexavalent chromium is mutagenic and carcinogenic in nature. Toxic effects may be passed on to children through the placenta. Chromium oxide is a strong oxidant and after dissolution it forms chromium acid, which corrodes the organs. The lethal dose is approximately 1-2 gm. Most countries apply a legal limit of 50 ppb chromium in drinking water. A professional illness in chromium industries is chromium sores upon skin contact with chromates. Chromium trioxide dust uptake in the workplace may cause cancer, and damage the respiration tract.
Common Cr(VI) removal technologies for drinking water applications are ion exchange, membranes, reduction/precipitation/coagulation/filtration, sorptive media etc. The trivalent chromium can be removed by contacting the solution with a weak acid cation exchange resin. The chromate can be removed by a weak base anion exchange resin in the presence of acid [Chopra, Randhir C, “Removal of chromium, chromate, molybdate and zinc” U.S. Pat. No. 3,972,810 (1976)]. Each of the resins requires different regenerate so that the process will require bulky equipment due to the requirement for separate sites for the regeneration of the two resins. Thus the chromium can be removed but the other pollutant is added to its solution. The most common industrial chromium treatment methods are reduction/precipitation/filtration. In this process, the Cr(VI) is reduced to Cr(III) typically by some reductant and chromium precipitated out as Cr(OH)3 and further coagulation were carried out with ferric salt and filtered. [Besselievre, E. B. (1969); The treatment of industrial wastes, McGraw-Hill, New York].
Reference may be made to Several U.S. Pat. Nos. 3,926,754; 4,036,726; and 4,123,339, which claims removal of hexa or trivalent chromium from wastewater electrochemically. In these patents, a process is described wherein wastewater containing hexavalent chromium ions is caused to flow between a plurality of electrodes. When the anode has a surface of iron/iron alloy/insoluble iron compound, an iron hydroxide derivative will be produced electrochemically. In this process, hexavalent chromium undergoes cathodic reduction to form trivalent chromium as insoluble chromic hydroxide, which complexes with iron at the anode The trivalent chromium compound, physically or chemically combine with the insoluble iron derivative to thereby permit removal from solution. The precipitate is then removed from aqueous by any conventional techniques.
Reference may be made to Jakobsen, K. and Laska, R. (1977) Advanced treatment methods for electroplating wasters, Pollution Engineering, 8:42-46] wherein aspearin as resin have been used in ion exchange for removal of chromium.
Another way of removing Cr(VI) from drinking water is to reduce the Cr(VI) to Cr(III) and precipitate it as chromium hydroxide. Reference may be made to El-Shafey. J. Phys. IV France, 107(2003) 419 wherein carbon sorbent has been used to remove Cr (VI) from aqueous solutions in the pH range 2.2-2.6.
Reference may be made to A. Li Bojic, M Purenovic and D Bojic, Water S A, 30(3), 2004, wherein micro-alloyed aluminium composite (MAIC) has been used as reducing agent for Cr(VI) removal from water. The mechanism of action is based on processes of reduction and co-precipitation by Al(OH)3, because Cr(VI) is removed from the water phase as metal chromium and insoluble Cr(OH)3 
Reference may be made to Liora Rosenthal-Toib et al., Synthesis of stabilized nanoparticles of zinc peroxide, Chemical Engineering Journal Vol 136, March 2008, wherein stabilized nanoparticles of zinc peroxide were prepared by an oxidation-hydrolysis-precipitation procedure. However the surface modifiers used in the present invention are different from the one reported in prior art.